1. Field of the Invention
This invention relates to an improvement in the maintenance, operation and results obtained from the use of ICP-MS, particularly in the semiconductor industry. Use of a nebulizer add-gas to reduce metal deposition on the sampling orifices of an inductively coupled plasma mass spectrometer (“ICP-MS ”) is disclosed. Specifically, dilute mixtures of Sulfur Hexafluoride (SF6) in an inert gas have been used to reduce transition metal deposition on the sampling orifices of an ICP-MS, thereby greatly enhancing the stability of the ICP-MS sensitivity over time without corroding the internal parts and/or chemically attacking the cones of the ICP-MS.
2. Description of Prior Art
To manufacture semiconductor-grade chemicals, exceedingly accurate detection of impurities is required, lest the resulting component materials be impure and thereby unreliable.
According to Perkin-Elmer, its ELAN ICP-MS technology originated in 1983 at the University of Toronto with Dr. D. Douglas and Prof. J. B. French, working under contract to the Sciex Division of MDS Health Group. Sciex continued development and sales until 1986, when a joint venture was formed between Sciex and the Perkin-Elmer Corporation. Perkin-Elmer describes that Sciex develops and manufactures the ICP-MS at the Sciex facility in Toronto, and Perkin-Elmer provides a worldwide sales and service network.
A schematic of the ELAN 6000 is shown in FIG. 1. As shown in FIG. 1(a), the sample is introduced into the plasma for destruction of the sample matrix and ionization. Ions pass from the torch region, at atmospheric pressure, to the quadrupole mass spectrometer, at vacuum pressures, through the interface region consisting of skimmer and sampling cones. The ion lens focuses the ions into the spectrometer, which separates the ions by mass-to-charge ratio and directs them to the detector where they are measured.
Metal complexes, alkoxides, and halides of transition metals such as Zr (Zirconium), Hf (Hafnium), Ta (Tantalum), Si (Silicon), Ti (Titanium), Cu (Copper) and Sb (Antimony) are being investigated for use as either high or low conductivity materials for the semiconductor industry. As such, the purity of these high conductivity materials is quite important. Typically these materials are decomposed in acids, bases or organic solvents and are subsequently analyzed by inductively coupled plasma mass spectrometry (ICP-MS) in order to obtain the concentration of trace elemental impurities.
However, owing to the high matrix concentration of the transition metal, deposition on the sampling orifices ultimately occurs which severely alters the analytical sensitivity and achievable detection limits of impurities present in these compounds. The degradation of performance can occur in as little as 10 minutes. Although dilution of the decomposed material is a viable alternative to decreasing the deposition on the sampling orifices, the requisite detection limits cannot be obtained.
Efforts to control the long-term, steady state operability of ICP detection apparatus have been reported in the literature. For example, D. Demers and A. Montaser describe the change in transport of analyte to plasma with change in injector gas flow rate for a concentric pneumatic nebulizer in their text, Inductively Coupled Plasmas in Ananytical Atomic Spectrometry, 2d Edition, ch. 11, at pages 524-525 (VCH). Specifically, to promote combustion and thereby reduce the background and noise in the plasma tailflame, oxygen is added to the injector gas flow. The introduction of oxygen has thus been used to decrease carbon deposition on the cones. Oxide formation must be avoided, however, to eliminate interferences from the erroneous detection of oxides.
In the article R. Hutton, et al., “Investigations into the Direct Analysis of Semiconductor Grade Gases by Inductively Coupled Plasma Mass Spectrometry,” Journal of Analytical Atomic Spectrometry, September 1990, v. 5 (pages 463-466), efforts to reduce matrix depositions on the sampler orifice of ICP-MS apparatus used to detect silane used to produce compounds of silicon are disclosed. Efforts described therein include the substitution of an alloy sample cone in place of the nickel cones then available, and supplementing the argon carrier gas with hydrogen gas. Thus, H2 is added to decrease the formation of Si deposition.
T. Jacksier, et al., “Qualitative Analysis of Arsine by Sealed inductively Coupled Plasma Atomic Emission Spectrometry,” Journal of Analytical Atomic Spectrometry, September 1992, v. 7 (pages 839-844) discloses the use of hydrogen, hydrogen chloride or chlorine as additive gases to promote arsenic vaporization. The reference describes reacting additive gases with the arsenic in an effort to form a volatile arsenic species that did not adsorb on the container walls. Additionally, Cl2 was added to reduce As deposition within the equipment.
For the forgoing reasons, there has been defined a long felt and unsolved need for a method of analyzing metal complexes, alkoxides, and halides of transition metals such as Zr (Zirconium), Hf (Hafnium), Ta (Tantalum), Si (Silicon), Ti (Titanium), Cu (Copper), Sb (Antimony) and the like that does not result in the deposition on the sampling orifices that degrades the analyzing capability of the apparatus while at the same time preserving the ability of the apparatus to provide data within the requisite detection limits as specified by the purities demanded in the electronics industry.